1. Field of the Invention
The present invention relates to a paper lateral edge detector, and more particularly, relates to a printer for forming a full-width print.
2. Explanation of the Prior Art
Recently, a digital camera is widely available. A user uses an image taken by the digital camera for a print by a color printer, additionally, for an observation on such as a personal computer. In the color printer, it is desirable to form a print without a margin surrounding the image, so-called a full-width print or non-margin print.
There are various types of color printers, such as an ink jet type, a thermal printing type, and a heat transfer type. For example, in a color thermal printer, a color thermal recording paper (hereinafter referred to as only a recording paper) having a yellow thermal coloring layer, a magenta thermal coloring layer, and a cyan thermal coloring layer is used. A light emitting element array on a thermal head is pressed to the recording paper during transport there of, and three colors of the thermal coloring layers are developed in sequence, to form a full color image on the recording paper.
Considering positional deviation and skewing in a widthwise direction of the recording paper when forming a non-margin print by the color thermal printer, the light emitting element array which is larger than recording paper in width is used. Moreover, when heat emitting elements not in contact with the recording paper are driven, waste heating occurs, so that it shortens useful life of the heat emitting elements. JPA No.9-272217 discloses a color thermal printer, in which the recording paper is prevented from skewing, to transport the recording paper in a straight line. Moreover, the lateral edge of the recording paper is detected by a CCD line sensor, to prevent waste heating of the heat emitting elements not in contact with the recording paper.
The aforementioned color thermal printer disposes the CCD line sensor upstream of a thermal head. The thermal head is extended in a sub scan direction, that is also a transporting direction of the recording paper. Accordingly, the lateral edge of the recording paper is detected at a considerable distance from a recording position where the heat emitting element array and the recording paper contact. In skewed manner of the recording paper, there is no coincidence in the lateral edges between a detecting position detected by the CCD line sensor and the recording position of the thermal head. Accordingly, in a non-margin print, there occurs a white stripe in the vicinity of the lateral edge and waste heating of the heat emitting elements. In a margin print, width thereof is not rendered uniform.
It is possible to principally solve the problem by disposing the CCD line sensor as near the heat emitting elements as possible. However, it is difficult to dispose the CCD line sensor very near the heat emitting elements since both the CCD line sensor and the thermal head have a size to some extent. If the CCD line sensor is disposed near the heat emitting elements, it causes a problem that output signal from the CCD line sensor is likely to become unstable according to a heating state of the heat emitting element array. Moreover, the CCD line sensor is relatively costly, thereby to increase manufacturing cost of the printer.